1. Field of the Invention
This invention relates to a flexible meshing type gear device of the type widely used in robot joints, semiconductor production equipment and the like, particularly to the shape of the teeth of a rigid internal gear and a flexible external gear used in the device.
2. Background Art
From the time of the invention of the flexible meshing type gear device by C. W. Musser (U.S. Pat. No. 2,906,143) up to the present, Musser and many other researchers, including the inventor of the invention described in this specification, have proposed various innovations. The variety of improvements in tooth profile proposed alone is considerable.
Regarding the tooth profile of the flexible meshing type gear device, the present inventor earlier proposed a tooth profile design method that uses rack-approximation of meshing between the teeth of the rigid internal gear and the flexible external gear to derive an addendum profile enabling wide range tooth meshing between the two gears (see JP-B 45-41171). The inventor also invented a method for avoiding tooth profile interference caused by rack approximation (JP-A 7-167228).
Specific tooth profiles developed by another include one that imparts an arc to the flexible external gear and uses the envelope curve of the arc as the tooth profile of the rigid internal gear (Japanese Patent No. 2,828,542). However, the efficacy of this invention is viewed as limited because use of an arcuate tooth profile was known earlier and the invention offers insufficient elucidation regarding the state of the envelope and other such interference issues.
There is a strong demand in the market for flexible meshing type gear devices that offer better performance. A particular need is felt for improvement in load capacity and prevention of tooth slipping (ratcheting) under excessive load condition.
Generally speaking, the main factors governing the load capacity of a flexible meshing type gear device are the rim strength of the flexible external gear tooth root and the strength of the rolling contact surface of the inner ring of the wave generator, particularly in the vicinity of the major axis. However, ability to resist ratcheting is closely related to tooth height.
An object of this invention is to enhance the load capacity of a flexible meshing type gear device by greatly enlarging the tooth height of the pass meshing profile, which is especially advantageous for increasing load capacity, and extending the overall tooth height without producing interference, thereby achieving higher ratcheting resistance, and by adopting negative deflection that reduces the amount of flexing of the flexible external gear and enables avoidance of meshing in the vicinity of the major axis where the bending stress of the tooth root rim is greatest.
The present invention achieves the foregoing object by giving the basic tooth profile of a flexible external gear having a flexing coefficient of smaller than 1 the shape of a convex arc and newly introducing a normal at the inflection point of the movement locus described by the center of the convex-arc tooth profile relative to the teeth of the rigid internal gear and an evolute of the locus, thereby clarifying the interference limit of the tooth profile and thus enabling maximization of the tooth height of the concave tooth profile portion of the rigid internal gear and the convex tooth profile portion of the flexible external gear, which perform main meshing, and maximization of the overall tooth height of the rigid internal gear without tooth profile interference.
Specifically,, the present invention relates to a flexible meshing type gear device having a rigid internal gear, a flexible external gear and a wave generator, wherein the flexible external gear includes a cylindrical body and external teeth formed on the outer surface of the cylindrical body, the wave generator flexes a cross-section of the external gear perpendicular to the gear axis into an elliptical shape to cause the external gear to mesh partially with the rigid internal gear, the rigid internal gear and the flexible external gear are both spur gears of module m, the number of teeth of the flexible external gear is 2n (n being a positive integer) fewer than the number of teeth of the rigid internal gear, and the wave generator is rotated to produce relative rotation between the two gears. The flexible meshing type gear device according to the present invention is characterized by the features set forth in the following.
A cross-section of the tooth trace of the flexible external gear perpendicular to the gear axis taken at an arbitrarily selected location is defined as a main cross-section. The difference at this main cross-section between the maximum radius of the neutral curve of the elliptical rim after deformation of the flexible external gear and the radius of the rim neutral curve before deformation is divided by mn. The result is defined as the flexing coefficient of the elliptical rim neutral curve. The flexing coefficient is set at a negative deflection smaller than a reference value of 1 to exclude the vicinity of the major axis of the ellipse from the meshing region of the teeth of the two gears.
The basic tooth profile of the flexible external gear is defined as a convex arc. The concave curve that the convex arc generates in the rigid internal gear is applied as the main portion of the tooth profile of rigid internal gear. The extreme point of the concave curve in the direction of the tooth crest is taken on a normal drawn at the inflection point of the movement locus described by the center of the convex arc tooth profile of the flexible external gear relative to the rigid internal gear. The point on the convex arc of the flexible external gear meshing with this extreme point is defined as the extreme point of the convex arc in the direction of the deddendum. The concave curve continuing on from the extreme point in the direction of the deddendum is applied as the deddendum of the flexible external gear.
The generated convex curve between the point on the normal drawn at the inflection point of the locus described by the center of the convex arc tooth profile of the flexible external gear and the return point of the generated curve appearing on the evolute of the locus is adopted as the tooth profile on the tooth crest side continuing on from the extreme point of the concave tooth profile of the rigid internal gear. A convex curve is further added to the generated convex curve without interfering with the flexible external gear.
In addition, the tooth crest point of the convex arc tooth profile of the flexible external gear is defined as the meshing point with the generated convex curve tooth profile of the rigid internal gear at the return point.
This configuration according to the present invention makes it possible to realize a flexible meshing type gear device with maximized tooth height wherein concave and convex tooth profiles effect pass meshing in the main cross-section of the flexible external gear apart from the major axis.
In addition to the ring-type flexible external gear composed of a cylindrical member having external teeth formed on its outer peripheral surface, there are also known cup-like and silk-hat-like flexible external gears. The flexible external gears of these shapes include a cylindrical body, an annular diaphragm continuous with one end of the body, an opening portion formed at the other end of the body, and external teeth for meshing with the aforesaid internal teeth formed on the outer peripheral surface of the body at the opening portion end. When this type of flexible external gear is used, the wave generator elliptically flexes cross-sections of the body of the flexible external gear so that the amount of flexing increases from the diaphragm toward the opening portion approximately in proportion to the distance from the diaphragm, thus partially meshing the external gear with the internal gear.
When necessary, relieving is applied to the teeth from the main cross-section of the flexible external gear toward the opening portion and toward the inner extremity continuing on from the diaphragm of the body.
It is also possible to eliminate meshing between the convex tooth profile of the rigid internal gear and the convex tooth profile of the flexible external gear by imparting clearance to the convex tooth profile of the rigid internal gear on the tooth crest side.
It is further possible to use the circle of curvature of the tooth profile at a typical point in the meshing region to approximate the generated convex tooth profile of the rigid internal gear.